1. Field of the Invention
This invention relates to medium to high carbon steel wires (content of C: 0.4 to 1.3%) which become products as cold worked without undergoing any further thermal treatments such as bluing and more particularly, to high strength and high toughness steel wires suitable as steel cord wires, wire saws, PC steel wire ropes and the like and also to a method for making the same.
2. Description of the Prior Art
For the manufacture of steel wires used as steel cord wires and various types of steel ropes, it is usual to subject a high carbon steel wire product to patenting, followed by drawing. Especially, cold drawing enables one to obtain a high strength steel wire. However, if the strength becomes too high as a result of the drawing, delamination may occur, accompanied by poor ductility and toughness. Thus, limitation is placed on the manufacture of high strength steel wires through drawing.
Under these circumstances, we made studies from the crystallographic standpoint at a level of nanometers for the purpose of developing high strength, high toughness and high ductility steel wires. As a result, it was found that by appropriately controlling cold working conditions and annealing conditions, carbides of a steel wire composed mainly of pearlite or bainite were successfully changed into cementite crystals whose diameter was on the order of nanometers (nm) (hereinafter referred to simply as "nano crystals"). It was also found that steel wires which had lamellar cementite consisting of the nano crystals as a carbide in the structure exhibited high strength, high toughness and high ductility. Our earlier Japanese Laid-open Patent Application 8-120407 was based on the above findings.
It will be noted that the interrelation between the plastic deformability of pearlite and the lamellar space has been known prior to the filing of the above-mentioned application. In the field of very fine wires such as steel cord wires, any technique of evaluating the state of existence of lamellar cementite has never been established. The relations between the existing form of cementite and the mechanical properties are not known in many respects. It is merely known from the experimental results of changing constituent compositions and preparation conditions according to the rule of trial and error that where the structure obtained after patenting consists of a fine, uniform pearlite structure, mechanical properties are good.
We have developed a steel wire wherein the crystal structure of lamellar cementite consists of nano crystals (which steel wire may be sometimes referred to simply as a nano crystal steel wire hereinafter) and can thus provide a high strength, high toughness and high ductility steel wire. It should be noted that there has been a demand for a steel wire having better mechanical characteristics and that developments of a steel wire which is high in strength and excellent in ductility have been expected.